Placoderms may have copulated in a similar way to sharks. Credit: The Museum of Victoria

Fish swimming 380 million years ago may have engaged in penetrative sex in a manner similar to modern sharks, a new study has revealed.

The finding, discussed in a _Nature_ video, also shows that reproduction by internal fertilization was much more widespread in early jawed vertebrates than was previously believed1.

John Long of Monash University in Clayton, Australia, and Kate Trinajstic of the University of Western Australia in Perth had previously been part of a team that discovered small plates of armour inside a number of fossilized armour-plated placoderm fishes2. The group showed that, rather than being dinner, these smaller plates were actually embryos (See: The oldest pregnant mum).

Now, along with Zerina Johanson of London's Natural History Museum, they show that the pelvic girdles of one species of extinct placoderm — Incisoscutum ritchiei — may have had long, articulated pieces of cartilage attached, which the fish could have used during copulation in a similar way to the claspers that male sharks use to penetrate females.

"We have an expression that humans like to get a leg over," Long says. "But these placoderms actually like to get a leg in."